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RP2040

Vias

  • Connect traces between different layers of the PCB. Similar terminology as holes in case design (through-hole VIAs => VIA that connects bottom and top side of PCB).
  • Generally better to have 1:1 mapping of vias to component: https://electronics.stackexchange.com/a/260106.

Decoupling Capacitor

  • tldr; helps ensure equilibrium with input voltages. Too high? => absorb excess energy. Too low? => provide more power to keep voltage stable.
  • VREG_IN will typically need a stronger capacitor (also listed in data sheet) than other capacitor-needing components, e.g. I/O pins.
  • Capacitors should be as close to the MCU as possible when doing PCB schematic/layout.

VREG_IN schematic

  • To get correct, generally just search for capacitor in RP2040 data sheet, then count the number of pin(s) in the Pinout diagram (search for pinout). By doing so, you get the following:
    • 6x IOVDD (digital IO supply) pins === 6x 100nF caps
    • 1x USB_VDD (USB supply) pin => "can use the same power source as IOVDD" => no need for an extra 100nF cap => 0x 100nF caps
    • 1x ADC_AVDD (ADC supply) pin => 1x 100nF cap
    • 1x VREG_IN (voltage regulator input supply) pin => 1x 1uF cap
  • 1x 10uF || 4.7uF pin => Lastly, most people typically add a stronger capacitor (10uF, or 4.7uF) to help smooth out low-frequency changes in the input voltage: https://www.autodesk.com/products/fusion-360/blog/what-are-decoupling-capacitors/.

VREG_OUT schematic

  • Sometimes you'll need to carefully read the pin descriptions to determine if they should be connected to VREG_OUT or VREG_IN.
    • 1x VREG_OUT pin => 1x 1uF cap
      • "The regulator must have 1μF capacitors placed close to its input (VREG_VIN) and output (VREG_VOUT) pins." (2.10.1 - Application Circuit)
    • 2x DVDD (digital core supply) pins === 2x 100nF caps
      • "...The connection between the output pin of the on-chip regulator (VREG_VOUT) and the DVDD supply pins is made off-chip, allowing DVDD to be powered from an off-chip power source if required..." (2.9.2 - Digital Core Supply (DVDD))
  • TODO: how do you determine where to put power / ground symbols in the capacitor circuitry?

Debug

  • Used for reprogramming the PCB in the event of a failure (don't interface through USB).
  • ATmega32U4 uses GPIO pins for this (thereby limiting pins for other use like RGB), but luckily RP2040 has its own dedicated pins for debug.